1. Field of the Invention
The present invention generally relates to receiver apparatuses and methods of processing received signals, and particularly relates to a receiver apparatus and method of processing a received signal with respect to a received signal including a plurality of channels.
2. Description of the Related Art
In recent years, the amount of digital information that is transmitted and received has been increasing in order to provide digital information having higher quality. As a method of transmitting a large amount of information efficiently, a QAM (quadrature amplitude modulation) method that transmits information by use of amplitude modulation and phase modulation is generally known.
For digital satellite broadcasting and the like, a QPSK (quadrature phase shift keying) is currently used that transmits and receives 4-value (2-bit) data per modulation by assigning four different phases to respective values. In the CATV system that is subjected to less disturbance, a QAM modulation/demodulation that modulates both amplitude and phase is used in order to achieve more efficient data transfer. The QAM modulation/demodulation represents information by assigning bits to both the amplitude changes and phase changes of a carrier wave, thereby achieving efficient data transfer with a limited band width.
Among such QAM modulation/demodulation, 64 QAM is mainly used at present, which transmits and receives 64-value information by use of 64 wave patterns having different amplitudes and phases. It is expected that, in the future, 256 QAM or 1024 QAM having further increased numbers of data values will be employed.
In order to achieve QAM modulation/demodulation using such a large number of data values, the transmission path, receiver, and the like are required to have a high SNR (signal-to-noise ratio). Accordingly, how to prevent the degradation of the SNR becomes a main issue when implementing a high-quality information transmission system.
FIG. 1 is a block diagram showing an example of the configuration of a related-art CATV receiver apparatus.
The CATV receiver apparatus of FIG. 1 includes a tuner unit 10 and a demodulation unit 20. The tuner unit 10 includes a band-pass filter 11, a first amplifier 12, a first mixer 13, a second amplifier 14, a third amplifier 15, a second mixer 16, a fourth amplifier 17, and VCOs (voltage controlled oscillators) 18 and 19 . Further, external SAW filters (surface acoustic wave filters) 30 and 31 are provided. The demodulation unit 20 includes an A/D converter 21 and a demodulator 22 .
The band-pass filter 11 passes only a range of effective CATV channels among the frequency components of the received signal. The first amplifier 12 situated at the next stage amplifies the output signal of the band-pass filter 11 to a desired level. The first mixer 13 multiplies the output signal of the first amplifier 12 by a local frequency generated by the VCO 18 , thereby performing frequency conversion. With this provision, a selected channel (desired wave) is positioned such as to correspond to the center frequency of the SAW filter 30 . The signal amplified by the second amplifier 14 is processed by the SAW filter 30 , so that frequencies (interfering waves) other than the desired wave are suppressed.
The third amplifier 15 amplifies the signal in order to compensate for the signal loss caused by the SAW filter 30 . The second mixer 16 multiplies the output signal of the third amplifier 15 by a local frequency generated by the VCO 19, thereby performing frequency conversion and generating a signal having an intermediate frequency (IF). This IF signal frequency is generally 36 MHz in Europe and 44 MHz in the United States. The SAW filter 31 is thus generally designed to have a center frequency corresponding to either one of these frequencies.
The desired wave converted into the IF signal is amplified by the fourth amplifier 17 for provision to the demodulation unit 20. In the demodulation unit 20, the A/D converter 21 performs A/D conversion with respect to the supplied signal so as to generate a digital signal. The demodulator 22 performs timing recovery, waveform equalization, discrimination processing, etc., with respect to the digital signal, thereby outputting a post-process signal.
The SNR of the received signal in the demodulation unit 20 varies depending on the IF signal frequency, the signal speed (symbol rate), and the sampling frequency of the A/D converter 21. In order to achieve an optimum SNR, therefore, there is a need to adjust the IF signal frequency, the signal speed (symbol rate), and the sampling frequency.
The signal speed (symbol rate) is determined according to the specification of the transmission-side system, and is not adjustable in the receiver side. The sampling frequency is specific to the A/D converter 21, and is basically a fixed value. In order to change the sampling frequency, there is a need to provide a special circuit configuration and oscillator for achieving such function, which is not desirable from a cost point of view.
The IF signal frequency is set such as to match the center frequency of the SAW filter 31, and cannot be variable. The SAW filter utilizes an acoustic wave that propagates as vibration energy concentrates on the surface of piezoelectric crystal, thereby allowing a signal having specific frequency to pass therethrough. This specific frequency cannot be made variable.
Accordingly, the configuration of the related-art CATV receiver apparatus as shown in FIG. 1 has a problem in that an optimum SNR cannot be attained.
Patent Document 1 discloses a configuration in which the need for the SAW filters for selecting frequency is eliminated in the receiver apparatus.
[Patent Document 1] Japanese Patent Application Publication No. 2000-092021.
Accordingly, there is a need for a receiver apparatus which can optimize the SNR of the received signal.